Method for installing optical fibres or cables in a tube using a fluid under pressure

ABSTRACT

Method for, using a fluid under pressure, installing optical fibers or cables in a tubular section comprising a supply tube and an intallation tube, each having an input and an output, the output of the supply tube being in connection with the input of the installation tube, a fluid under pressure being fed near the input of the supply tube, and the cable being conducted into the input of the supply tube and being propelled through the tubular section by the entraining force of the fluid, at least part of the fluid being discharged from the supply tube at the output of the supply tube and, at the input of the installation tube, a second fluid under pressure being fed. Due to said measures, the pressure drop at the input of the tubular section may be overcome without utilizing mechanical means which might damage the fiber or cable.

[0001] The invention relates to a method for installing optical fibresor cables, using a fluid under pressure, in a tubular section comprisinga supply tube and an installation tube, each having an input and anoutput, the output of the supply tube being connected to the input ofthe installation tube, a fluid under pressure being fed near the inputof the supply tube, and the cable being conducted into the input of thesupply tube, and being propelled through the tubular section by theentraining force of the fluid.

[0002] Such a method is disclosed, e.g., in EP-A-0 108 590. In thismethod, an optical cable is conducted into the tube by way of mechanicalforce, particularly by way of a pair of pressure rolls. To prevent lossof pressure, the input of the tube or of the device is provided, by wayof pressure rolls, with a sealing preventing the escape of fluid alongthe cable or fibre. The pressure rolls serve to overcome the pressuredifference between the environment and the space where the fluidpressure is prevailing upon introduction of the cable. In EP-A-0 292037, a similar method is disclosed, the pressure rolls exercising aforce on the cable which exceeds the one required to overcome thepressure difference. With the additional pushing power, a greaterinstallation length may be achieved than with only the entraining forceof a fluid, particularly in the event of thicker cables.

[0003] For overcoming the force required to introduce the cable into thespace where the fluid pressure is present, it is also known to make useof a fluid flowing with increased velocity over a first part, viewedfrom the input, of the tube length. Exercising mechanical forces, suchas in the event of thin or vulnerable fibres or cables, is undesirableon account of the possibility of mechanical damage.

[0004] In EP-A-0 345 043 it is disclosed to feed the pressure to the legof a T-shaped coupling piece, the ongoing portion of the T receiving thecable at one end, and at the other end being connected to a supply tubeextending over a relatively modest length, e.g., 1 m, into theinstallation tube and having a diameter which is 0.1 to 0.8 times theone of [sic] the diameter of the installation tube. In the narrow supplytube, the velocity of the fluid flow is a great deal higher than in theinstallation tube, as a result of which the required tensile force isgenerated. A drawback of said solution is that a relatively largeportion of the total pressure difference between the input and theoutput of the installation tube acts on the narrow supply tube, so thateither the presssure must be chosen very high for sufficient residualpressure to remain for the actual installation, or the installationlength is restricted. The first case often encounters practical, safetyand cost problems, and the second is always undesirable, unless theinstallation is effected over a modest length only.

[0005] In EP-A-0 287 225 a method is disclosed, an additional flow rateof the fluid over an initial portion of the installation tube beingrealised as well. Here, the diameter of the installation tube is thesame as the one of the supply tube, and the latter is in fact part ofthe installation tube. The additional fluid flow is realised by, at theend of the supply tube, where it changes into the installation tube,blowing off part of the fluid by way of an adjustable valve. Saidsolution has the problem that the velocity of the fluid in the supplytube soon achieves the velocity of sound. In the event of air as afluid, and a supply tube having a length of several meters, such mayalready be the case. When achieving the velocity of sound, the energy ofthe fluid is no longer converted into entraining forces on the cable butinto acceleration of the fluid itself.

[0006] An additional problem, larger in practise, is that a relativelythin cable in a relatively wide supply tube with respect to said cable,in the event of a high fluid velocity will soon start to twist,so-called buckling, as a result of which the cable will prematurelystick in the tube and can no longer be propelled by the fluid. In theevent of narrow supply tubes, such drawback does not exist, but thedrawbacks mentioned earlier do.

[0007] The object of the invention is to provide for a method enablingthe installation of an optical fibre or cable using a fluid, a fluidflow also being applicable for overcoming the pressure differencebetween the space outside the installation tube and the inside thereof,the fluid flow, over a first portion of the tubular section, having ahigher velocity than over the remaining portion of the installationsection, without the problems described above occurring.

[0008] For this purpose, the invention provides for a method of theaforementioned kind, characterised in that, at the output of the supplytube, at least part of the fluid from the supply tube is discharged, andthat, at the input of the installation tube, a second fluid is fed underpressure.

[0009] The invention also provides for a device for, using a fluid underpressure, installing optical fibres or cables in a tubular sectioncomprising a supply tube and an installation tube, each having an inputand an output, the output of the supply tube being in connection withthe input of the installation tube, means being provided for, near theinput of the supply tube, feeding a fluid under pressure, and for meansto conduct the cable into the input of the supply tube, the cable beingpropelled through the tubular section by the entraining force of thefluid, characterised in that, at the output of the supply tube, meansare provided to discharge at least part of the fluid from the supplytube, and that means are provided for feeding, at the input of theinstallation tube, a second fluid under pressure.

[0010] Essentially, the fluid is preferably completely discharged fromthe supply tube. In addition, the fluids for the supply tube and theoutput tube preferably come from one and the same source.

[0011] Below, the invention will be explained in more detail on thebasis of exemplary embodiments with reference to the drawing. Therein:

[0012]FIG. 1 shows a schematic view of an installation for, using afluid under pressure, installing a cable into a tube according to afirst prior art;

[0013]FIG. 2 shows a schematic view of an installation for, using afluid under pressure, installing a cable into a tube according to asecond prior art;

[0014]FIG. 3 shows a schematic view of an installation for, using afluid under pressure, installing a cable into a tube according to afirst aspect of the invention;

[0015]FIG. 4 shows a schematic view of an installation for, using afluid under pressure, installing a cable into a tube according to asecond aspect of the invention;

[0016]FIG. 5 shows a schematic view of an installation for, using afluid under pressure, installing a cable into a tube according to athird aspect of the invention;

[0017]FIG. 6 shows a schematic view of an installation for, using afluid under pressure, installing a cable into a tube according to afourth aspect of the invention;

[0018]FIG. 7 shows a schematic view of an installation for, using afluid under pressure, installing a cable into a tube according to afifth aspect of the invention;

[0019]FIG. 8 shows a schematic view of an installation for, using afluid under pressure, installing a cable into a tube according to asixth aspect of the invention; and

[0020]FIG. 9 shows a schematic view of an installation for, using afluid under pressure, installing a cable into a tube according toexample 1 [sic].

[0021] In the figures, corresponding parts are designated by the samereference numeral, preceded by the number of the figure, possiblycompleted by a zero.

[0022] Furthermore, the following notation is used in the description ofthe figure:

[0023] D_(c)=cable diameter;

[0024] D_(d)=internal diameter of the installation tube;

[0025] D_(p)=internal diameter of the supply tube;

[0026] l_(p)=length of the supply tube;

[0027] l_(d)=length of the installation tube.

[0028]FIG. 1 shows an installation for introducing a cable 101 by way ofa supply tube 102 into an installation tube 103. By way of a connection104, a fluid, e.g., air, is fed under pressure. Part of said fluid isblown off at the end of the supply tube 102 by way of a valve 105. Aninstallation of this type is described in EP-A-0 287-225. In the eventof said installation, the diameter of the supply tube is the same as theone of the installation tube. Below, it will be investigated to whatextent the ratio of said two diameters affects the installation process.

[0029] EP-A-0 108 950 describes that, if a cable having a diameter ofD_(c) is blown into a tube having a diameter of D_(d), the total blowingforce of F_(b) at a pressure difference of Δp is given by:$\begin{matrix}{F_{b} = {\frac{\pi}{4}D_{d}D_{c\quad \Delta}p}} & (1)\end{matrix}$

[0030] The hydrostatic force F_(hs) which must be overcome to introducethe cable into the pressure space over a pressure drop of Δp then is:$\begin{matrix}{F_{h\quad s} = {\frac{\pi}{4}D_{c\quad \Delta}^{2}p}} & (2)\end{matrix}$

[0031] From (1), (2) and FIG. 1, for a pressure of p₀ at the input sidenear connection 104, an atmospheric pressure p_(a) at the output of theinstallation tube, and beyond the valve 105, and an intermediatepressure at the location of the valve 105, which is set to Pi, it may bederived that the hydrostatic force F_(hs) in the supply tube 102 havinga diameter of D_(p) is precisely cancelled out if: $\begin{matrix}\begin{matrix}{{\frac{\pi}{4}D_{c}{D_{p}\left( {p_{0} - p_{i}} \right)}} = {\frac{\pi}{4}{D_{c}^{2}\left( {p_{0} - p_{a}} \right)}}} \\{{o\quad {r:}}\quad} \\{{p_{i} = \frac{{\left( {D_{p} - D_{c}} \right)p_{0}} + {D_{c}p_{a}}}{D_{p}}}\quad}\end{matrix} & (3)\end{matrix}$

[0032] A supply tube which is relatively narrow with respect to thecable diameter of D_(c) gives a relatively high flow velocity, andtherefore a relatively large pressure drop over a short piece of tube isfeasible, provided the velocity remains sufficiently below the velocityof sound, without air needing to be discharged by way of valve 105. Alarge portion of the pressure, however, is “consumed” to overcome thehydrostatic pressure.

[0033] Example: A cable having a diameter of D_(c)=3 mm must be blowninto a tube having a diameter of D_(d)=5.5 mm. A supply tube having adiameter of D_(p)=4 mm makes a pressure of p₀ drop from 10 bar(absolute) to a p_(i) of only 3.25 bar.

[0034] In order to retain a good deal of the pressure to blow into thecable, it is precisely a wider supply tube which is desired, as is shownin FIG. 2. Now, partially blowing off the fluid is certainly required todevelop sufficient entraining forces over a short length. If, e.g., adiameter of D_(p)=15 mm is chosen, an amply sufficient pressure ofp_(i)=8.2 bar will remain to blow into the cable. Here, however, theproblems described above with respect to buckling and exceeding thevelocity of sound by the flowing fluid do arise.

[0035] From (1), (2) and FIG. 3, which shows an exemplary embodimentaccording to the invention, a separate fluid under pressure being fed tothe installation tube by way of the input of an engaging piece 306 whichis coupled to the installation tube 303, it may now be derived that thepressure drop in the supply tube 302 is precisely cancelled out if:$\begin{matrix}{{{\frac{\pi}{4}\left( {{D_{c}D_{p}} - D_{c}^{2}} \right)\left( {p_{p0} - p_{p1}} \right)} = {\frac{\pi}{4}{D_{c}^{2}\left( {p_{o} - p_{a}} \right)}}}{{o\quad {r:D_{p}}} = {\frac{p_{p0} + p_{0} - p_{p1} - p_{a}}{p_{p0} - p_{p1}}D_{c}}}} & (4)\end{matrix}$

[0036] where:

[0037] p_(p0)=the pressure at the input of the supply tube,

[0038] p_(p1)=the pressure of the discharge opening of the supply tube.

[0039] The net pushing force F_(push) of the “flow motor”, i.e., thesupply tube, then is: $\begin{matrix}{F_{p\quad u\quad s\quad h} = {\frac{\pi}{4}\left( {{D_{c}D_{p}} - D_{c}^{2}} \right)\left( {p_{p0} - p_{p1}} \right)}} & \text{(4a)}\end{matrix}$

[0040] For p_(p0)=p₀ and p_(p1)=p_(a), equation (4) becomes:

D_(p)=2D_(c)  (4b)

[0041] In the event of a diameter of the supply tube 302 which is twiceas large as the one of the cable 301, the complete pressure thereforestill being available for blowing in. If for the supply tube a largerpressure is used than for blowing in, the diameter of the supply tubemay be reduced still further. This may also, as is shown in FIG. 4, beachieved by applying several supply tubes 402, 402′ in cascade, therebeing completely blown off, by way of valves 405, 405′, at the end ofeach supply tube.

[0042] For n cascaded supply tubes, there may be derived a more generalversion of formula 4 for the internal diameter D_(p) of the supplytubes. Said more general formula reads: $\begin{matrix}{D_{p} = {\frac{p_{p0} + {p_{0}/n} - p_{p1} - {p_{a}/n}}{p_{p0} - p_{p1}}D_{c}}} & \left( 4^{\prime} \right)\end{matrix}$

[0043] For the diameter of the supply tubes, it applies more in generalthat D_(p)=(1+1/n) D_(c).

[0044] The problems occurring in the installation methods described withreference to FIGS. 1 and 2 may therefore be overcome with the methodaccording to the invention by a suitable dimensioning of the supplytube.

[0045] A further advantage obtained by the invention is that it ispossible to work with different fluids, since the supply tube isdisconnected from the installation tube. There may therefore be applieda fluid having a higher viscosity in the supply tube. As a result, theflow rate in the supply tube may be restricted still further and inaddition very short lengths may be achieved, as a result of which theprobability of buckling may in fact be excluded.

[0046] An example of a fluid for application in the supply tube ishydraulic liquid. If the sealing between the supply tube and thepressure space in the engaging piece 306 or 406, which is coupled to thetube to be installed, is not complete, then part of the fluid from thesupply tube may be introduced together with the cable. In this case, theliquid for the supply tube must be compatible with cable and tubematerial and, in addition, not adversely affect the friction coefficientbetween cable and tube. A safer fluid for the supply tube is water or,better still, a lubricant. This may be the same lubricant as the oneused when installing the cable. In this manner, the lubricant may evenbe applied beautifully evenly. If it is undesirable that fluid comesfrom the supply tube into the installation tube, the cable may be wipedclean in the space between the supply tube and said pressure space.

[0047]FIG. 5 shows a special embodiment. Here, the supply tubes 502,502′ are placed inside a engaging piece 507. Such is particularlyadvantageous if there are cascaded engaging pieces, and the fluid may bedirectly obtained from the pressure space within the engaging piece.

[0048] It is also feasible to apply several cascaded loose supply tubes602, 602′, distributed over a section, such as is shown in FIG. 6. Saidsupply tubes each deliver a slight pushing force and operate as “flowmotors”. If there are many in a row, in this way an even distribution ofthe pushing forces may be achieved. Flow in the tube between the “flowmotors” is still present, but not critical. It is therefore alsopossible to use narrow installation tubes. The feed and discharge ofboth the “flow motors” and the installation tubes may be taken care ofthrough tubes 608 and 609 which lie along the installation tubes 603.Said feed and discharge tubes may have a larger diameter than theinstallation tube, so that a small pressure drop may be achieved overthe first-mentioned tubes. In addition, various installation tubes maybe operated from the tubes 608 and 609. A reduction of the dimensions ofa bundle of installation tubes may be effectively realised in thismanner.

[0049]FIG. 6 shows a cable 601 being installed in a tube 603. Several“flow motors” are cascaded to effect the propulsion of the cable 601.The heart of a flow motor is the supply tube 602, 602′. In such tube, afluid flows with a velocity in excess of the one of cable 601. The fluidis fed by way of a tube 608 and discharged by way of a tube 609. Theengaging piece 604 forms the connection between feed tube 608 and supplytube 602. The disconnecting piece 611 forms the connection betweensupply tube 602 and discharge tube 609. In the coupling pieces 604 and611, bulkheads 612 and 613 respectively, provide for the cable 601 notbeing pressed too much against the wall. For the installation tube 603,engaging pieces 606, 606′ and disconnecting pieces 615, 615′ are used,which have their input and discharge from and into, respectively, thetubes 608 and 609.

[0050] The application with cascaded “flow motors” particularly offersadvantages if a cable already runs through said “motors”. If the cableis still to be introduced, each time upon entering into the supply tubea counterforce for overcoming the pressure drop will be experienced.This is compensated only if the cable has penetrated far enough into thesupply tube. There are solutions, however, to this problem:

[0051] Making use of a previously installed pulling rope.

[0052] Providing the supply tube with a venturi, such as in EP-A-0 318280, (see the description of FIG. 8 below) or of an input piecegenerating a so-called coanda spiral flow, as is described in EP-A-0 508016.

[0053] Applying a supply tube having a great discharge leak, in such amanner that there is hardly any pressure built up, and the cable maytherefore enter easily. Only at the end of the supply tube, the leak isplugged by the cable blocking the leak (see the description followingbelow of FIG. 7).

[0054]FIG. 7 shows a cable 701 being installed in a tube 703. Only oneof the “flow motors”, with which the propulsion of the cable 701 isrealised, is shown. The heart of a flow motor once again is the supplytube 702 having a fluid therein flowing with a velocity in excess of theone of cable 701. The fluid is fed by way of tube 708 and discharged byway of tube 709. The connection 710 to the engaging piece 704 now is asomewhat narrower tube. The dimensions are such that “in operation” (seelater) the flowing fluid may achieve sufficient velocity to exercisesufficient propelling force onto the cable. Initially, the dischargeflow, by way of the supply tube 702, is the disconnecting piece 711 a,through which the larger part of the medium flows, and a tube 716 a,which forms a connection between the disconnecting piece 711 a and thedischarge tube 709, is so large that by way of the narrow tube 710 thereis hardly built up any pressure in connecting piece 704. The cable 701may then easily be conducted into the flow motor. While filling thesupply tube 702 with cable 701, the pressure already starts to increasesomewhat, but the cable 701 also receives an entraining force over anever larger length. Once it has arrived at the opening 717, between thedisconnecting pieces 711 and 711 a, the pressure suddenly becomes evenhigher. After all, the cable 701 fits essentially precisely into theopening 717, so that the flow towards disconnecting piece 711 a is nowblocked. The discharge flow now runs by way of disconnecting piece 711and by way of a narrower tube 716. Now, the situation has been achievedthat the “flow motor” works normally. In the connecting pieces 704 and711, bulkheads 712 and 713, respectively, once again provide for thecable 701 not being pressed too hard against the wall. For the feed andthe discharge, respectively, of the fluid towards or from theinstallation tube, engaging piece 706 and disconnecting piece 715 areused, which have their feed and discharge in the same tubes 708 and 709.

[0055]FIG. 8 shows a cable 801 which is installed in a tube 803 andfurthermore in a tube 803 a. One of the “flow motors” is shown, withwhich the propulsion of the cable 801 is realised. The heart of a flowmotor once again is the supply tube 802/802 a, in which a fluid isflowing having a velocity larger than the one of the cable 801. Thefluid is again fed by way of tube 808. By way of connecting tube 810,which has such a course that no turbulence causing an additional flowresistance occurs, the fluid is introduced into the coupling piece 804.Here, the fluid is conducted evenly into a supply tube 802 designed as aventuri. The diameter of tube 802 is much smaller than the one of tube808, namely, so much smaller that the velocity of the fluid increases somuch that the pressure becomes so much lower (according to Bernoulli)that it effectively becomes about equal to the one at the end of tube803. There is then no longer any pressure drop at the point where thecable 801 enters the coupling piece 804. Upon the input, the cable 801is shielded, by the shielding bulkhead 812, from fast-flowing fluid inthe transverse direction. In the supply tube 802, the heart of theventuri, an entraining force is exercised on the cable 801. The samealso occurs in the conically broadening part 802 a of the supply tube.In tube portion 802 a, the velocity of the fluid is once again graduallyreduced, and simultaneously the pressure increases again. Thedimensioning should be such that the cable 801, when running throughtube portion 802 a, has built up sufficient entraining forces to becapable of compensating at any rate the pressure drop at the input inthe coupling piece 804. This may be determined, e.g., by simulation orby trial and error. At the bulkheads 813 of the “disconnecting piece” ofthe supply tube, the fluid is discharged once again. The net pushingpower of the “flow motor” is the built-up entraining force in tubeportions 802 and 802 a, and in the “disconnecting piece” havingbulkheads 813. It is possible to have the bulkheads 813 run straightover a more extended section, before the fluid is discharged. Thedischarge of the fluid is now directly outwards, however, within aprotection tube 818. The protection tube 818 in this case serves as areturn channel. The cable 801 is conducted through a conic end 819 ofthe “disconnecting piece” having bulkheads 813 into the continuationpiece 803 a of the installation tube 803. The conic end 819, afterpassing through of cable 801, may form a close sealing between the “flowmotor” and tube 803 a. For the installation tube, coupling piece 806 isused, which obtains its feed from the tube 808. The installation tubehas its discharge to protection tube 818 by way of a same type ofbulkhead 813 a as at the disconnecting piece of the supply tube.

EXAMPLE 1

[0056] A small cable having a diameter D_(c) of 3 mm, a weight W of0.1N/m and a rigidity B of 0.01 Nm² is installed in a tube having adiameter D_(d) of 4 mm and having a change [sic] coefficient f=0.2between tube and cable. Blowing in with a pressure of 10 bar (absolute)and an additional pushing force of 10 N is possible over a distance of518 m (standard section, each time having a right-angle curve after 200m). With water as a fluid, a length of 1078 m may be attained. Withoutadditional pushing, a length of 1005 m is still attained. When, however,the force for overcoming the pressure drop at the cable input is notcompensated, said length decreases strongly.

[0057] For the embodiment according to FIG. 1, in the event of equalD_(p) and D_(d) in the supply tube, a large part of the availablepressure is required to compensate the input force. This is why a muchlarger D_(p)=15 mm must be chosen. From (3), there then follows ap_(i)=8.2 bar.

[0058] If, for said geometry, air is applied as a fluid, at a length ofthe supply tube which is restricted to several meters, the velocity ofsound is exceeded. The energy of the air flow in that case is dissipatedby a shock wave, and not by the entraining effect on the small cable.Therefore, a liquid must be applied, e.g., water. The velocity of theliquid is limited to several dozens of meters per second and the volumeflow to several liters per second.

[0059] The drawback of a supply tube having a large diameter—cablebuckling—is still present. With the embodiment according to FIG. 3, saidproblem is solved and with the one according to FIG. 4, still better.From formula 4, upon equal input and output pressure at the supply tubeand the installation tube, there follows a diameter DP for the 6 mmsupply tube.

[0060] For the embodiment of FIG. 3, air may once again be used in theinstallation tube. In addition, larger diameters are then feasible forthe installation tube, the supply tube simply remaining equal. With theembodiment according to FIG. 1, such is definitely unthinkable.

EXAMPLE 1a

[0061] Fibres are blown into installation tubes (guide tubes) having thesame dimensioning as in Example 1, which tubes are installed as a bundlein a protection tube. Blowing refers to 518 m per unit. To attain 2 km,therefore, 4 units in cascade are required (instead of two units, as inFIG. 4). Said units may be fed using a separate feed tube, while theoutput is possible either separately, as in FIG. 7, or through theprotection tube (as in FIG. 8, but not necessarily with a tubeimplemented as a venturi). The diameter of the feed tube must be largerthan the one of the installation tubes, such in order to restrict thepressure losses over the feed tubes; for the protection tube, such isalready automatically the case. If, e.g., the diameter of the feed tubeis just as large as the one of the installation tube, over a feed lengthof 500 m, up to the first flow motor, for the same flow rate in theinstallation tube as in Example 1, the entire available pressure wouldalready have been consumed in the feed process. The pressure gradient inthe feed tube fortunately decreases rapidly with increasing diameter. Inthe event of the volume rate remaining equal, the pressure gradient isinversely proportionate to D_(d) ^(19/4), D_(d) in this case relating tothe internal diameter of the feed tube (see Appendix A below). Adiameter of 6 mm already suffices and, using a somewhat larger diameter,e.g., of 8 mm, several installation tubes may also be simultaneouslyexcited.

[0062] The method described above may be applied using, e.g., thefollowing steps. First lay a protection tube having, e.g., a length of 2km and a diameter of 50/40 mm (externally/internally). Subsequently blowin a bundle of tubes, consisting of 1 (supply) tube having a diameter of10/8 mm and 12 (installation) tubes having a diameter of 5/4 mm, usingthe method as described in EP-A-0 785 387. After installation of thebundle of tubes, the protection tube must be opened, e.g., each 500 m.In the open spots, “flow motors” as shown in FIG. 8 may now be mountedbetween the installation tubes and be connected to the supply tube.Thereafter, the protection tube is closed again. This may be effectedthrough previously shifted tube pieces having a larger diameter and therequired couplings, or using divisible housings which may be placedafter the “flow motors”. After said actions, the cables may be blown in.It is also feasible, with one feed tube, to feed flow motors which areconnected to parallel installation tubes.

EXAMPLE 2

[0063] A cable having a diameter D_(c) of 10 mm is installed into a tubehaving any diameter through a method according to FIG. 3 (in theinstallation tube, installation is preferably effected using compressedair having a pressure of about 7-10 bar, as in EP-A-0 292 037).Installation using a liquid, e.g., water, is of course also possible.The supply tube has a diameter D_(p) of 14 mm. As a result, a lubricant,such as, e.g., Polywater, having a dynamic viscosity p of 2 Pa, isallowed to flow at a pressure p_(p0) of 50 bar. According to (4a), said“flow motor” delivers a force of 154 N, sufficient as a mechanical forcewhen pushing/blowing according to EP-A-0 292 037. At a length l_(p) of50 cm, from (5) and (6) there follows a volume flow Φ_(v) of 2 l/s and avelocity v of 26 m/s. The liquid, therefore, flows considerably fasterthan the cable (condition for delivering the entraining force) and avolume flow of 2 l/s may be delivered using conventional pumps. In thissituation, the flow is turbulent, as was assumed in the calculations.

[0064] In order to prevent a possibly too large generation of heat inthe lubricant for said combination of flow rate and pressure difference,it may be necessary to take measures for this purpose, e.g., conductingthe flow of Polywater through a cooled heat exchanger before it isreturned to the pump. In addition, the volume flow may still be reducedby choosing a narrower supply tube. In such case, it is necessary tocascade various supply tubes in the flow motor. In spite of saidcascading, the total of the volume flow becomes smaller with respect tothe one at a flow motor having a single supply tube. Including thevelocity with which the liquid flows, by the way. This is no objection,however, as long as said velocity remains amply above the velocity ofthe cable (0.5 to 1 m/s).

EXAMPLE 3

[0065] A small cable having a diameter D_(c) of 3 mm is installed in atube having any diameter, through a method according to FIG. 5. Thesupply tubes have a diameter D_(p)=3.6 mm. Water is used as a fluid,both for the supply tube and for the installation tube. The pressurep_(p0) equals 6 bar. According to (4a), one “flow motor” delivers aforce of 0.71 N. Compensation of the pressure drop when introducing itinto the installation tube (also 6 bar) requires 5 units. To compensatefor friction losses, it may make sense to use a sixth supply tube. For alength l_(p)=2.5 cm, from formula (5) and (6) (see Appendix A) therefollows a volume flow Φ_(v)=0.09 l/s per supply tube and a velocity v=29m/s. The liquid therefore flows considerably faster than the cable, anda volume flow of 0.55 l/s in total may be delivered by a good (domestic)tap. From (6) and (7), there follows a Reynolds number of 38267; theflow therefore being sufficiently turbulent, as was assumed in thecalculations.

[0066] If the piping must remain dry such as, e.g., upon installation ina building, an embodiment according to FIG. 4 may also be used,(compressed) air being fed to the installation tube. Likewise, flushingwith (compressed) air after installation with water, according to FIG.5, is possible. In either case, however, the solution loses theadvantage that no special provisions are required. Still, flushing withair may also be achieved by utilising the pressure of the water mains byway of a buffer vessel, as in FIG. 9.

[0067]FIG. 9 shows an application of Example 3. A cable 901 from a reel901 a is installed in a tube 903 using water. The water is obtained froma tap and is conducted, by way of feed hose 908, to the housing 907 forthe flow motors having supply tubes 902 and 902 a. The feed hose 908 issplit up into 2 paths, 908 a and 908 b, the latter of which goes by wayof a buffer vessel 921. Initially, path 908 b is blocked by tap 922. Theenclosed air in the buffer vessel will be compressed by the pressurefrom the tap, as a result of which the water in the buffer vessel 921will rise to a specific height. Apart from the housing 907 for the flowmotors, the water will also feed the coupling piece 906 which, in FIG.5, is integrated with the housing 507, to the tube 903. The flow motorshaving supply tubes 902 discharge their water into tube 920, which runsby way of a discharge hose 909 to, e.g., a washbasin 925. The water fromtube 903, which is a much smaller amount, may be caught in, e.g., abucket. Once the cable 901 is installed, there follows a flushing usingair. This is effected by now blocking the path 908 a with the tap 922and opening path 908 b. The air in the top of the buffer vessel 921,which is at the same pressure as the water, will then take over the roleof the water. To prevent the water from escaping into tube 908 b, afloat 924 is placed in the top of the buffer vessel 921. In order not to“spill” a large part of the limited amount of air enclosed in the flowmotors, housing 907 is closed off with a tap 923.

1. Method for, using a fluid under pressure, installing optical fibresor cables in a tubular section comprising a supply tube and aninstallation tube, each having an input and an output, the output of thesupply tube being in connection with the input of the installation tube,a fluid under pressure being fed near the input of the supply tube, andthe cable being conducted into the input of the supply tube and beingpropelled by the entraining force of the fluid through the tubularsection, characterised in that, at the output of the supply tube, at anyrate part of the fluid is discharged from the supply tube and that, atthe input of the installation tube, a second fluid under pressure isfed.
 2. Method according to claim 1, characterised in that the outputsof the supply tube and the installation tube flow out into the sameenvironment.
 3. Method according to claim 1 or 2, characterised in thatthe input of the supply tube and the input of the installation tube arecoupled to the same fluid source.
 4. Method according to any of theclaims 1 to 3 inclusive, characterised in that the fluid for the supplytube consists of a gas.
 5. Method according to claim 4, characterised inthat the fluid for the supply tube consists of air.
 6. Method accordingto any of the claims 1 to 3 inclusive, characterised in that the fluidfor the supply tube consists of a liquid.
 7. Method according to claim5, characterised in that the fluid for the supply tube consists ofwater.
 8. Method according to claim 5, characterised in that the fluidfor the supply tube consists of hydraulic liquid.
 9. Method according toclaim 5, characterised in that the fluid for the supply tube consists ofa lubricant.
 10. Method according to claim 5, characterised in that thefluid for the supply tube consists of a gas compressed to a liquid. 11.Method according to any of the claims 1 to 3 inclusive, characterised inthat the pressures on the inputs of the supply tube and the installationtube are equal.
 12. Method according to any of the claims 1 to 3inclusive, characterised in that the fluid flowing through theinstallation tube consists of air.
 13. Method according to any of theclaims 1 to 3 inclusive, characterised in that the fluid flowing throughthe installation tube consists of water.
 14. Method according to any ofthe claims 1 to 3 inclusive, characterised in that the fluid flowingthrough the installation tube consists of a gas compressed to liquid.15. Method according to any of the claims 1 to 3 inclusive,characterised in that the pressure at the end of the supply tube isgreater than, or equal to, the pressure at the start of the installationtube.
 16. Device for, using a fluid under pressure, installing opticalfibres or cables in a tubular section comprising a supply tube and aninstallation tube, each having an input and an output, the output of thesupply tube being in connection with the input of the installation tube,there being provided for means near the input of the supply tube to feeda fluid under pressure, and for means to conduct the cable into theinput of the supply tube, the cable being propelled through the tubularsection by the entraining force of the fluid, characterised in that, atthe output of the supply tube, there is provided for means to dischargeat least part of the fluid from the supply tube and there being providedfor means to, at the input of the installation tube, feed a second fluidunder pressure.
 17. Device according to claim 16, characterised in thatthe supply tube consists of n cascaded sections.
 18. Device according toclaim 17, characterised in that the means to feed a second fluid underpressure to the input of the installation tube consist of n sections.19. Device according to claim 17 or 18, characterised in that thediameters of the supply tubes are about equal to, or greater than,$\frac{\left. {p_{p0} + p_{0}} \middle| {n - p_{p1} - p_{a}} \middle| n \right.}{p_{p0} - p_{p1}}$

times the diameter of the cable.
 20. Device according to claim 19,characterised in that the diameters of the supply tubes are about equalto, or greater than, (1+1/n) times the diameter of the cable.
 21. Deviceaccording to any of the claims 16 to 20 inclusive, characterised in thatthe supply tube or supply tubes are mounted fixedly to the installationtube.
 22. Device according to any of the claims 16 to 20 inclusive,characterised in that the supply tube or supply tubes are located in theinside of the means to, at the input of the installation tube, feed asecond fluid under pressure.
 23. Device according to claim 22,characterised in that feeding fluid for the supply tube or supply tubesis directly coupled to the inside of the means to feed a second fluidunder pressure to the input of the installation tube.
 24. Deviceaccording to claim 17, characterised in that the means to discharge atleast part of the fluid from the supply tube and the means to feed asecond fluid to the input of the installation tube comprise a dischargetube and a supply tube, respectively, which tubes extend parallel to theinstallation tube.
 25. Device according to claim 24, characterised inthat the discharge tube surrounds the supply tube and the installationtube.
 26. Device according to claim 17, characterised in that the meansfor, at the input of the installation tube, feeding a second fluid,comprise a feed tube having an internal diameter which is larger thanthe internal diameter of the installation tube, which feed tube servesfor feeding fluid to several supply tubes, which are each connected toone or more parallel installation tubes.